1. Field of the Invention
This invention relates to a communication system, and in particular to a covert communication system capable of two way covert voice and data communication undetectable by radio frequency band electronic surveillance equipment. The invention also relates to a system for covert approach guidance, including voice and data communication, during aircraft refueling and instrument landing operations, and to a covert means of ground to ground and ground to air communications.
2. Description of Related Art
The ever-increasing sophistication of modern electronic surveillance equipment has made it difficult to hide radio emissions emanating from communications equipment. Spread spectrum and special modulation or encryption techniques can be used to deny unauthorized listeners the information content of the transmissions, but do not prevent detection of the existence of the transmissions. Covert millimeter wave transceivers capitalize on the absorptive properties of the atmosphere to limit the range at which detection can occur, but still use the radio frequency spectrum and are therefore still subject to intercept by modern high gain extended frequency range electronic surveillance sets.
In order to avoid the above problems, it has previously been proposed to use light as a medium for covert communications. An early attempt to provide a covert communication system using light as the medium of transmission was disclosed in U.S. Pat. No. 2,858,421. The system of U.S. Pat. No. 2,858,421 used a rare gas electrical discharge tube with radio frequency or pulse excitation of the gas, the excitation signal being modulated by conventional AM, FM, or other modulation techniques, Although infrared radiation was preferred, the system was also described as being adaptable for use in the ultraviolet region of the electromagnetic spectrum, by providing a mercury vapor lamp UV source. However, difficulties in implementation, in particular in connection with providing for the necessary high frequency on and off switching of the source tube, prevented this type of system from attaining widespread usage.
Nevertheless, despite such problems, light wave communication systems have continued to be proposed, for example in U.S. Pat. No. 3,657,543 (modulated excitation of an optically emissive diode, especially in the near-infrared region), U.S. Pat. No. 4,079,246 (line of sight communication and reception of scattered light using a high powered laser source), and U.S. Pat. No. 4,493,114 (UV communication in the "solar blind" spectrum, from 0.23 to 0.28 micrometers, using a pulse controlled mercury argon source). Each of these systems suffers either from the above-mentioned implementation problems or from the drawback that the output power of ultraviolet laser sources are currently too low for practical line of sight communication, and therefore none has achieved widespread usage.
Of course, numerous light communication systems are also known in the context of fiber optic cable communications (see e.g., U.S. Pat. No. 4,703,474), but such systems cannot be used in applications such as air-to-air communications. Despite the above proposals, therefore, a need still exists for a practical line of sight communication system using light, and in particular ultraviolet light, as the medium of transmission.